1. Technical Field
The present invention relates generally to frequency modulators and, in particular, to a frequency modulator having a variable carrier frequency. A variable carrier frequency enables the frequency modulator to be used in a variety of communication modes.
2. Description of Related Art
In general, the discrimination between the carrier frequency of a video signal and the carrier frequency of an audio signal is made with reference to the carrier frequency of the video signal as a main carrier, and the carrier frequency of the audio signal as a sub carrier. In very high frequency (VHF) and ultra high frequency (UHF) wireless communications, video signals are transmitted in an amplitude modulation (AM) transmission mode and audio signals are transmitted in a frequency modulation (FM) transmission mode. The carrier frequency of an audio signal and the carrier frequency of a video signal vary with the transmission modes.
A National Television Systems Committee (NTSC) mode and a Phase Alternation Line (PAL) mode are the most representative transmission modes. In the NTSC mode, the carrier frequency of an audio signal is 4.5 MHz higher than that of a video signal. In the PAL mode, the spacing between the carrier frequency of an audio signal and the carrier frequency of a video signal is 5.5 MHz in a B/G mode, 6.0 MHz in an I mode and 6.5 MHz in a D/K mode. Therefore, frequency modulators which can be commonly used in a variety of transmission modes have been developed.
FIG. 1 is a schematic block diagram illustrating a conventional frequency modulator. In the frequency modulator, the oscillation frequency of a voltage controlled oscillator (VCO) which receives an audio signal AUDIO and generates an output signal FM_OUT is determined by a plurality of external inductor-capacitor tanks L1-C1, L2-C2, L3-C3 and L4-C4. The operations of the inductor-capacitor tanks L1-C1 through L4-C4 are controlled by switches SW1, SW2, SW3 and SW4, respectively. Each of the inductor-capacitor tanks L1-C1 through L4-C4 generates a control signal VCO_CTRL to control the oscillation frequency of the VCO. The switches SW1, SW2, SW3 and SW4 are selectively turned on in response to a switch selection signal SW_SEL. The switch selection signal SW_SEL is set externally or is set by an internal microcomputer depending on the broadcasting mode. A frequency modulator such as that shown in FIG. 1 requires many parts that are disposed external to an integrated circuit (IC), thereby increasing the manufacturing cost and complicating the manufacturing processes of the frequency modulator.
FIG. 2 is a block diagram illustrating a conventional frequency modulator that uses a gyrator. The frequency modulator of FIG. 2 includes an audio amplifier and a sub carrier selector to alter the inductance of an inductor Lg. Unlike the frequency modulator of FIG. 1, the frequency modulator of FIG. 2 uses a gyrator as an inductor, the gyrator being an element of an inductor-capacitor tank. The gyrator is built in an IC.
The oscillation frequency of a voltage controlled oscillator is determined by the inductor Lg, which is realized as a gyrator and a capacitor C. The inductance of the gyrator is set by a channel selection signal CH_SEL that depends on the broadcasting mode. Then, an audio signal AUDIO is amplified by an audio amplifier and alters the inductance of the gyrator. When the inductance of the gyrator is altered, the resonant frequency of the inductor-capacitor tank also changes, as does an output signal VOUT. A voltage controlled current source converts voltage to current because an input signal to the gyrator is current. An amplifier connected to the output node of the frequency modulator compensates for current loss in the gyrator.
The frequency modulator of FIG. 2 reduces the number of devices external to an IC. However, the frequency modulator of FIG. 2 generates a significant amount of noise because the noise characteristic of the gyrator is very poor. Accordingly, there is a need for a frequency modulator which reduces noise and is capable of being adapted to a variety of broadcasting modes.
To solve the above and other related problems of the prior art, a low noise frequency modulator is provided which is capable of being adapted to a variety of broadcasting modes. Further, a modulation index compensation circuit is provided for adjusting the variation of the output frequency of a frequency modulator according to a carrier frequency. Moreover, a voltage controlled oscillator is provided for use in a frequency modulator which is capable of being adapted to a variety of broadcasting modes.
According to an object of the invention, there is provided a frequency modulator having a variable carrier frequency. The frequency modulator includes a voltage controlled oscillator for frequency-modulating an oscillator input signal using an oscillation frequency of the voltage controlled oscillator as the carrier frequency. The oscillation frequency being set by a set signal. A phase/frequency detector detects phase and frequency differences between an output signal of the voltage controlled oscillator and a predetermined reference signal. A low pass filter receives an output of the phase/frequency detector and generating the set signal. An audio signal amplifier amplifies an audio signal and generates a pair of output signals that have voltage levels that change in opposing directions. A modulation index compensation circuit, connected to an output node of the audio signal amplifier, changes the voltage levels of the pair of output signals and outputs the pair of output signals having the changed voltage levels to the voltage controlled oscillator as the oscillator input signal to adjust a frequency variation of the output signal of the voltage controlled oscillator according to the oscillation frequency that is set by the set signal.
According to another aspect of the invention, in a frequency modulator that includes an audio signal amplifier for amplifying an audio signal and generating a pair of output signals that have voltage levels that change in opposing directions and a voltage controlled oscillator for frequency-modulating the pair of output signals using an oscillation frequency set by a predetermined set signal as a carrier frequency, there is provided a modulation index compensation circuit. The modulation index compensation circuit includes at least one voltage level adjustment circuit between an output node of the audio signal amplifier and a predetermined reference voltage. The operation of the at least one voltage level adjustment circuit is controlled according to a selection signal. The at least one voltage level adjustment circuit is connected in parallel with other voltage level adjustment circuits when there is two or more voltage level adjustment circuits. The modulation index compensation circuit changes the voltage levels of the pair of output signals to adjust a frequency variation of an output signal of the voltage controlled oscillator according to the oscillation frequency that is set by the predetermined set signal.
According to yet another aspect of the invention, there is provided a voltage controlled oscillator for frequency-modulating an input signal using an oscillation frequency of the voltage controlled oscillator as the carrier frequency. The oscillation frequency is set by a predetermined set signal. The voltage controlled oscillator includes an oscillator core for generating an output signal which changes depending on the predetermined set signal. A capacitor converts the output signal of the oscillator core into a current and outputs the current. An input stage controls an oscillator current drained from the oscillator core using an input signal pair and the current output from the capacitor.